The present invention relates to a procedure for controlling a heat treatment, particularly for annealing embrittled reactor vessels.
During the normal operation of a nuclear reactor, the reactor vessel, which is normally made of steel and which houses a core containing nuclear fuel, is exposed to intense radiation. Experience has shown that this radiation causes changes in the fine grain structure of the steel walls of the vessel. These structural changes make the walls brittle, a problem commonly referred to as reactor vessel embrittlement. Embrittlement reduces the flexibility of the vessel wall and increases the susceptibility of the vessel wall to fracturing, particularly if subjected to sudden stresses, such as due to operating transients and pressurized thermal shock events.
Because of this embrittlement phenomenon, the United States Nuclear Regulatory Commission requires that a reactor vessel be removed from service when embrittlement reaches a predetermined stage, thus ending the useful life of this portion of the nuclear power plant. Replacement of such a vessel is extremely expensive because the vessel is built into and is a part of the reactor containment building, thereby making replacement economically impractical.
In order to deal with this problem, it has been proposed to subject such a vessel to annealing in place in order to restore the ductility and toughness of the metal constituting the reactor vessel.
In order for such an annealing treatment to be successful, the vessel wall must be heated to defined temperatures, must be precisely maintained at each defined temperature for a predetermined period of time, and must undergo temperature changes at a controlled rate. Because of the size of a reactor vessel and in view of the relatively high temperatures employed for such an annealing treatment, heating of the vessel wall involves a number of variables and uncertainties which must be taken into account when making heater power adjustments. In particular, there are inherent delays between any change in heater power and the resulting change in the temperature of the vessel wall and the heat produced by any one heater can influence the vessel in a manner which is not completely predictable.